Studies have continued on the role of tropomyosin suppression in neoplastic transformation. To ascertain which, if any, components of the transformed phenotype are directly caused by suppression of tropomyosin synthesis, specific inhibition of translation of tropomyosin mRNA in intact cells was attempted by constructing an anti-sense producting vector and introducing it into NIH 3T3 cells. Successful transfectants were selected by resistance to the antibiotic, G418, and these have now been cloned. By translation of mRNA in cell-free systems and by northern blot analysis, it was established that tropomyosin suppression results from reduced levels of specific tropomyosin mRNAs. To improve the specificity of molecular probes for mouse cell tropomyosin mRNA and DNA, and to obtain more effective antisense sequences, a cDNA library derived from NIH 3T3 mRNA was produced in the lambda-gt11 expression vector. This library was screened with the TM4 cDNA probe and with anti-tropomyosin antiserum to select specific cDNA clones for the various species of tropomyosin. Thirty TM4-positive plaques were identified and purified.